When conducting machining operations on a particular workpiece, it is important to determine the correct RPM of the cutting tool which is based upon surface foot rating of the material constituting the workpiece. It is well known that the surface speed of the material, calculated in a surface foot rating, has a definite relationship with respect to the diameter of the cutting tool and the RPM of the cutting machine. Various tables have been developed showing this relationship. Therefore, utilizing these tables for specific surface speed would convey information relating to the proper diameter of the cutting tool and the speed of rotation of the cutting tool. Once this information was determined, the operator of the machine would select the diameter of the cutting tool, insert it into the machine, then would manually set the proper RPM of the spindle to which the cutting tool is attached.
Furthermore, if the operator wishes to perform several successive operations on the workpiece utilizing tools of different diameters, the proper RPM must be calculated or read from the table for each of the tools. For example, if the operator wishes to drill a one-half inch diameter hole in a material, he must calculate the RPM based on the diameter of the cutter and the surface foot rating of the material to be machined. Subsequently, if the operator wishes to drill a one-inch diameter hole in the same material, the operator must change the RPM of the spindle, based upon the calculation of the surface feet per minute of the material to be machined. Not only would these calculations have to be performed, they must be retained by the operator so that he would not have to calculate the RPM of the spindle if the same operations were to be performed on more than one workpiece.
Most manual knee-type milling machines utilized at the present time employ two methods of changing the spindle speed (RPM). The first method would include a mechanical variable-speed drive in which the operator changes the RPM by a crank handle which, in turn, changes the belt position on a bellows-type pulley arrangement. A mechanical dial associated with the milling machine would display the particular spindle speed. Unfortunately, although the particular speed of the spindle would be displayed, the actual spindle speed could vary substantially, based upon the mechanics of the design.
The second method which is presently utilized for changing spindle speeds would employ a step pulley. Although this step pulley would provide a more accurate spindle speed, it is limited to a selection of only several discrete RPMs.
A number of prior art patents have issued relating in general to the problem of determining the proper operating parameters for a particular endeavor. For example, U.S. Pat. No. 2,942,781 issued to Abatemarco relates to calculating and analyzing devices which are adapted to utilize data obtained from standard tables to determine optimum operating conditions. In a machine tool such as a lathe or milling machine, the fundamental relationship between various parameters such as time, length of cut, feed rate, rotary speed of the work or cutter, the number of teeth on the cutter, the chip load per tooth per revolution as well as the relative surface speed of the cutter and work and the diameter of the cutter or work are known. Therefore, if one or more of these parameters are inputted into the standard data analyzer shown in the Abatemarco patent, parameters dependent upon those which were introduced into the analyzer can then be determined. However, the calculator device of the Abatemarco patent has no control over the actual speed of a milling machine spindle.
Similarly, U.S. Pat. No. 4,707,793 issued to Anderson describes a method of determining the feed rate and cutting speed for cutting metal by inputting various parameters into a calculator. Any calculations produced by the Anderson calculator would have to be manually inputted into a particular machine.
U.S. Pat. No. 5,136,903 issued to Hibi describes a method of automatically determining a machining range such as the best chucking point on a workpiece machine utilizing a CNC program. However, as was true with respect to the Anderson and Abatemarco patents, this patent does not deal with the peripheral speed of a rotating cutter used in a milling machine.
U.S. Pat. No. 4,698,773 issued to Jeppsson is directed to a system for adaptively limiting the feed rate of a milling machine's positioning axes during a milling operation. Strain gauges are employed on a numerically controlled milling machine which senses the load on the cutting tool. A signal is then sent to a device for automatically overriding the program feed rate to maintain a peak side load on the cutting tool. As was true with respect to the above-noted patents, the Jeppsson device does not control the speed of the spindle but only overrides the feed rate.